The forward derailleur guides and positions the chain on two or three chain rings. The rear derailleur performs a similar job on six or eight pinions. There result substantial lateral displacements of the chain and this increases the risk of placing the chain in torsion with the drawbacks that result therefrom (poor performance, chain disengagement, wear on parts, etc.). This same increase in the number of relative ratios gives rise to a greater complexity in the choice of these latter and errors are frequent (gear changing involving a large increase in effort, error in the direction of manipulation of the shift levers, calculation necessary before changing speed, a need to control visually the position of the chain to know the action to take).
Devices have already been proposed tending to solve these different problems but none has solved all of them.
For example, FR-A-2,530,573 has for its object a device for controlling two derailleurs, comprising two actuating members each connected to two derailleurs by a transmission member, the device comprising a manipulating member of one of the actuating members connected to the other actuating member by a lost motion connection.
This arrangement permits, by a single control member, controlling simultaneously the two derailleurs and the lost motion connection provided in the transmission permits offsetting the control of the pedal derailleur relative to the control of the free wheel derailleur.
Such a device solves the problem of misalignment of the chain but gives rise to a jolt in pedaling due to the amplification of the abruptness of changing the gear by a change of pinion in the same direction.
U.S. Pat. No. 4,412,828 describes a simultaneous control device for a front derailleur with two chain rings and a rear derailleur with five pinions by means of a system of cams and cam followers. This device is adapted to actuate step by step the rear derailleur to the central pinion and change of gear while remaining on this central pinion, then actuating step by step for the remaining pinions. This known device uses in practice half the pinions for each respective level. It is difficultly adaptable to modern derailleurs with three levels and more than five pinions. Moreover, the simultaneous control is continuous, which gives rise to a long path for the control member and bulkiness of the assembly.
U.S. Pat. No. 4,412,828 describes a simultaneous control device for derailleurs, for example with two levels and five pinions, ensuring ten ratios by this combination. This system, which requires practically alternate changes of level with the simultaneous passages from one or several pinions, ensures a transmission ratio varying in a continuous manner. This device, which would be very complicated for a large number of gear ratios, does not concern itself with the angle of the chain.
Moreover, all the known devices are controlled by a shift lever or a handle which can occupy as many positions as there are usable speeds, such that the cyclist must pay attention to the effective position of the shift lever before actuating it in one direction or the other. Moreover, as indicated in connection with U.S. Pat. No. 4,412,828, such a device is bulky because of the long path of the control member.